Method of making static-resistant detonator plug



Sept. 20, 1966 5 ET AL 3,274,318

4 METHOD OF MAKING STATIC-RESISTANT DETONATOR PLUG Original Filed Feb. 6, 1962 INVENTOR-S United States Patent f 3 274,318 METHOD OF MAKTNG STATIC-RESISTANT DETONATOR PLUG Matthew L. Spillane, Tomaqua, and George J. Heirnberger, Lehighton, Pa., assignors to Atlas Chemical Industries, Inc., Wilmington, Del., a corporation of Delaware Original application Feb. 6, 1962, Ser. No. 171,421, now Patent No. 3,194,160, dated July 13, 1965. Divided and this application Get. 3, 1964, Ser. No. 409,340

3 Claims. (til. 264-104) This application is a division of -S.N. 171,421, filed February 6, 1962, and now U.S. Patent 3,194,160, granted July 13, 1965.

The present application relates to an improved semiconductive protective plug suited to use in an electric detonator to provide a means by which the electric detonator is made highly resistant to accidental firing by static electricity, and to an electric detonator which includes the improved plug. The present application also relates to a method of producing the improved semi-conductive protective plugs.

Electric detonators commonly available in commerce generally comprise a cylindrical shell of metal having one open end and containing a detonating charge, a heat-sensitive ignition composition and an electric firing means embedded in, or in contact with, the ignition composition. The det-onating charge usually comprises a primer and/or a flash charge in combination with a detonating base charge. The firing means normally utilized is a bridge wire of high electrical resistance which joins two leg wires. The leg wires extend through a plug which seals the open end of the shell and are adapted to be connected to a source of electrical energy. When sufilcient electrical current is passed through the bridge wire, it becomes heated and activates the ignition composition. The ignition composition fires the primer or flash charge which, in turn, carries the detonation to the base charge.

The danger of extraneous electrical energy, particularly in the form of static electricity, has long been recognized. The ignition compositions generally used in commercially available detonators are highly heat sensitive and exposure of the bridge wire to a static discharge is quite capable of igniting the ignition composition and subsequently firing the detonating charge.

Although the danger from accidental firing of detonators by extraneous electricity is present in almost all blasting operations, the danger is particularly present when the blasting operation is carried out under conditions conducive to high levels of atmospheric static electricity, for example, in electrical, dust or snow storms. Atmospheric static conditions are especially dangerous in seismo'graph blasting operations because such operations are frequently carried out in areas which are highly susceptible to atmospheric static.

A number of solutions to the problem of static susceptibility of electric detonators are known, and most solutions have been effective at least to some degree. However, none have been found to be entirely satisfactory when cost, ease of manufacture, effectiveness, and storage life are al considered.

Among the simplest and most effective means of protecting an electric detonator from premature initiation by extraneous static charges is a semi-conductive plug positioned within the detonator shell in intimate contact with bared portions of the leg wires and extending to close proximity with the inside shell wall. The plug is spaced from the shell wall by a distance substantially less than the distance between either of the leg wires and the shell wall in the locus of the ignition composition. Thus, a static charge is conducted to the edge of the semi-conduc- 3,274,318 Patented Sept. 20, 1966 tive plug-body and allowed to group, or discharge to the shell wall. In this way the static charge is safely grounded to the shell outside the locus of the ignition composition. The term semi-conductive is used herein to describe the property of a material to act as an insulator at low voltages, for example, from 10 to volts, but act as a low resistance material when exposed to high voltages comparable to those required to initiate a detonator by a charge of static electricity. The semi-conductive plug presents a conductive path for high voltage, static discharges and a high resistant path to a low voltage of the type used normally to fire the detonator.

A typical protective plug of the prior art is a semiconductive body made up of a wax matrix and including a quantity of finely divided conductive material therein. Examples of the finely divided conductive materials generally used are powdered aluminum, carbon, galena, brass or copper. A serious disadvantage of the prior art protective plugs is the difficulty of maintaining intimate contact between the leg wires and the protective plug, particularly during the detonator manufacturing process wherein the various component parts are assembled. Ruptures or cracks extending along the interface between the leg wires and the protective plug reduce the area of intimate contact between the leg wires and the plug. This condition markedly and adversely affects the efficiency of the plug to afford protection against static charges. Prior art plugs are often brittle and susceptible to cracking or splitting when the leg wires therein are subjected to a very slight pull or tug of A to /2 pound. It is most difficult to assemble a detona-tor containing a prior art protective plug and not expose the leg wires positioned through the plug to slight tugs suificient to crack the plug. Prior to the present invention, it was thought that the amount of finely divided conductive material in the plug was limited to a maximum of 70% by weight. Although plugs containing greater amounts of conductive material were desirable, they were not practical to fabricate because they suffered from a serious lack of support from the matrix material and are so structurally weak that they are impractical. The amount of conductive materials incorporated in prior art protective plugs is also limited by the thin body of the matrix. The term body describes the ability of a specific volume of matrix material to hold the conductive material in suspension. This latter property of the matrix is particularly important in the forming or casting Operations carried out during fabrication of the plug.

A protective plug of the present invention comprises a finely divided conductive material admixed in a matrix of polyethylene. Low melting polyethylene (about 300 F.) has been found eminently satisfactory as the matrix material. In a preferred form a plug of the present invention contains powdered aluminum as the conductive material in an amount greater than 70% of the total weight of the plug.

The present invention contemplates a method of manufacturing protective plugs which comprises the steps of admixing a finely divided conductive material with a melt of low melting polyethylene, and allowing the resulting mixture to harden. In a preferred form of the invention, the hardened mixture of polyethylene and finely divided conductive material is subjected to a grinding operation and the ground material pressure formed, while heated to a temperature above the melting point of the polyethylene component, to form a cohesive compacted mass. Preferably the finely divided conductive star-ting material is of a size which will pass a 325 mesh USS screen and the low melting polyethylene material utilized as the plug matrix has a melting point of about 300 F. In a preferred form of the invention wherein the hardened mixture of polyethylene and finely divided conductive material is ground, the ground material has a size which will pass an 8 mesh USS screen. Although the protective plug may be pressure formed with holes therein to accept detonator leg wires, it is preferred that the plug be pressure formed directly on the leg wires. A ground polyethylene-powdered aluminum mixture is excellently suited as a feed material for an injection mold press. The method of the present invention facilitates the production of a structurally strong plug containing over 70% by weight of conductive material.

Now looking in detail at the protective plug: the plug has a matrix of polyethylene, preferably having a melting point of about 300 F. A product of Eastman Chemical Products, Inc. sold under the trade name of Epolene C is a low molecular weight polyethylene having a specific gravity of about 0.907 and a viscosity at 300 F. of approximately 9,000 centipoises and is eminently suited to use as the matrix material. The polyethylene matrix is placed in a suitable container and heated until it is molten.

The finely divided conductive material is then slowly added to the melt with constant stirring. The mixture is then removed from the heat and allowed to cool. The product is a hardened mass of polyethylene containing a finely divided conductive material distributed therethrough. This product may then be ground and the ground material compacted by means of heat and pressure into a hard plug of polyethylene containing a finely divided conductive material distributed therethrough. Copper, brass, carbon, aluminum and galena are examples of suitable finely divided conductive materials suited to use in the present invention. The plug contains at least 70% by weight of the finely divided conductive material and in a preferred embodiment a plug of the present invention contains powdered aluminum in the ratio of 80% by weight of aluminum to 20% by weight of polyethylene.

In the following table the structural strength of typical prior art protective plugs compounded of powdered aluminum and wax are compared with protective plugs of identical size and assembly of the present invention. In the table Tests 1 through 5 show the test results of prior art plugs comprised of 60% by weight of aluminum and 40% by weight of wax and Tests 6 through show the results of a similar test on plugs of the present invention containing 80% by weight aluminum and by weight polyethylene. The prior art plugs were made by melting 40 weight parts of Acrawax C. Beads G582 (a synthetic wax produced by Glyco Products (30., having a melting point of between 137l39 C., a flash point of 230 C. and a specific gravity of 1.04), and adding 60 weight parts of powdered aluminum to the melt with constant stirring. The molten mixture was then cast into a mold to form a plug. The mold utilized had two bared leg wires spaced about /s" apart extending therethrough so that the plug was formed around the leg wires. After the mixture solidified the mold was removed. The plugs of the pres ent invention were made by melting 20 weight parts of polyethylene having a melting point of about 300 F. and adding 80 weight parts of powdered aluminum to the melt with constant stirring. The mixture was allowed to cool to form a hardened mass. The hardened mass was then ground to about a size which would pass an 8 mesh USS screen. The ground material was fed into an injection press at about 420 F. and the plug was pressure formed at about 700 psi. around a pair of bared leg wires spaced about /8" apart. In each of the following tests a plug assembly consisting of a protective plug having two leg wires extending therethrough and in intimate contact therewith was exposed to a force sufficient to crack the plug. The force was applied to the plug by exerting a pull on the leg wires extending from one side of the plug in opposite directions, i.e. in directions which tended to further separate the wires. A separating force was exerted on the leg wires until the plug cracked and the pounds pull recorded. For example, in Test 1 a prior art plug cracked when a separating force of /2 pound was TABLE I Plug Composition Pounds pull (percent by weight) Test N0. required to crack plug Powdered Wax Poly- Al ethylene A detonator in accord with the present invention will be more particularly illustrated by reference to the accompanying longitudinal drawing. The drawing shows a detonator having a shell 11 which is fabricated of a suitable metal, for example, copper. Shell 11 has an open end and a closed end. A base charge 12 is positioned in the closed end of shell 11. A suitable base charge is pentaerythritoltetranitrate. Atop the base charge is a primer charge 13 of an explosive such as mannitol hexanitrate. Finally a flash charge 23 composed of 75% by weight of diazodinitrophenol and 25% by weight of mannitol hexanitrate is positioned atop the primer charge 13. A perforated cup 24 is mounted in an inverted position over the charges. A match assembly 14 comprised of plates 15 and 16 separated by insulation 10, bridge wire 17 and ignition composition 18 is positioned within the shell in firing relation to flash charge 23. Lead mononitroresorcinate is a suitable ignition composition. A dielectric sleeve member 19 is positioned around match assembly 14 to maintain the match assembly out of contact with shell 11. Leg wires 20 and 21 are electrically connected to plates 15 and 16 respectively and extend through sealing plug 22. Sealing plug 22 is suitably fabricated of rubber. A semi-conductive protective plug 9 containing about by Weight of aluminum in a polyethylene matrix is positioned in intimate contact with leg wires 20 and 21 at a point between match assembly 14 and sealing plug 22.

Although an instantaneous electric detonator has been used to illustrate the protective plug of the present invention, it will be appreciated that the plug is adapted to use in any type of electric detonator or squib.

Th following table shows the results of a series of comparative electrostatic tests conducted using detonators containing protective plugs of the prior art and detonators containing protective plugs of the present invention. Test Series 1 shows the results of tests conducted on prior art detonators, and Test Series 2 shows the results of similar tests on the detonators of the present invention. The tests were made to determine the sensitivity of the detonators to an electrostatic charge. Each detonator of each series was separately tested by electrically connecting each detonator to a condenser previously charged by a high voltage power source. In carrying out the test, the shunted leg wires of the detonator were electrically connected to a source of condenser discharge, the shell of the detonator was electrically connected to a ground source, and incremental increasing amounts of condenser energy was applied to the shunted leg wires until detonation of the detonator occurred. The minimum condenser energy required to fire the detonator or, if the detonator failed to fire, the maximum charge which could be delivered from the test equipment was recorded. For example, in Test Series 1 shown in the following table, 2.5 prior art detona-tors each containing a protective plug comprised of 60% by weight of powdered aluminum in a wax matrix were tested one at a time by connecting the detonator shell to ground, shunting the leg wires and connecting the shunted leg wires by means of a suitable switch to a condenser previously charged from an electrical source. In Test Series 1, the condenser having a maximum available capacitance of 0.10 mfd. was charged from a high voltage source. The high voltage source utilized could be varied by 1 kv. increments between and kv. The condenser was initially charged from the 10 kv. source and discharged through the detonator to be tested. If the detonator did not fire, the condenser was charged fromthe 11 kv. source and the detonator again tested. This procedure was repeated until all of the detonators fired or until the capacity of the equipment was reached. In Test Series 1, all 25 detonators fired before a maximum charge could be delivered by the condenser when charged from a 13 kv. source. The highest energy level required to initiate any one detonator in the group Was 8.4 joules. The lowest energy level sufficient to fire one in the group was 1.5 joules. The average amount of electrical energy required to fire the series was 5.1 joules. Test Series 2 shows the results obtained when 25 detonators of the present invention containing protective plugs made up of 75% by Weight of powdered aluminum in a polyethylene matrix were tested in a similar manner to those in Test Series 1. It will be noted that 11 detonators of Test Series 2 did not fire when exposed to an electrical energy level of 11.2 joules, which was the capacity of the static test equipment. The figure for the average energy level required to fire the detonators in Test Series 2 is given as 9.4+ joules. This figure was arrived at by utilizing the actual firing data from the 13 detonators which fired at 111.2 joules or belew, and utilizing 11.2 joules, the capacity of the test equipment, as the energy level required to fire the remaining detonators. It will be noted that l l detonators of the 25 tested in Test Series 2 did not in fact fire at 11.2 joules, thus the actual energy level required to fire the series would be higher than 9.4 joules shown in Table II. Test Series 3 and 4 were carried out in a similar manner and the average energy level required to fire the detonators was calculated on the same basis as Test Series 2.

Having thus described the invention, what is claimed is:

1. A method of producing a protective plug which comprises the steps of admixing finely divided aluminum with a melt of polyethylene, said aluminum present in amounts of from about to about by weight of the total mix, allowing the resultant mix to harden, grinding the hardened mix, and pressure forming by injection molding at a temperature above the melting point of the polyethylene to form a cohesive compacted mass.

2. The method of claim 1 wherein the hardened mixture is ground to a particle size which will pass an 8 mesh USS screen.

3. The method of claim 1 wherein the ground hardened mixture is pressure formed about and in intimate contact with two bared detonator lead wires.

References Cited by the Examiner UNITED STATES PATENTS 2, 422,809 6/ 1947 Stup akofi et al. 2643I'29 2,577,923 12/1951 Scott et al. 2 641 26 XR 2, 828,35 1 3/ 1958 Rade 264-104 XR 2,855,630 10/1958 Veley 264-111 XR 2,877,501 3/1959 Bradt.

FOREIGN PATENTS 48 9,981 8/ 1938 Great Britain. 604,695 7/ 1948 Great Britain.

ROBERT F. WHITE, Primary Examiner.

J. A. FINLAYSON, Assistant Examiner. 

1. A METHOD OF PRODUCING A PROTECTIVE PLUG WHICH COMPRISES THE STEPS OF ADMIXING FINELY DIVIDED ALUMINUM WITH A MELT OF POLYETHYLENE, SAID ALUMINUM PRESENT IN AMOUNTS OF FROM ABOUT 70 TO ABOUT 80% BY WEIGHT OF THE TOTAL MIX, ALLOWING THE RESULTANT MIX TO HARDEN, GRINDING THE HARDENED MIX, AND PRESSURE FORMING BY INJECTION MOLDING AT A TEMPERATURE ABOVE THE MELTING POINT OF THE POLYETHYLENE TO FORM A COHESIVE COMPACTED MASS. 